During surgery, particularly in the field of cardiovascular surgery, patients are subject to the risks of extracorporeal circulation, also calle cardiopulmonary bypass (CPB), most notably thromboembolism and bleeding. The CPB circuit itself is very thrombogenic and patients must be fully anti-coagulated, usually with heparin. Maintenance of a safe level of anti-coagulation is patient-dependent and is periodically evaluated with repeated measures of serum heparin concentration activated clotting time, and in some cases platelet aggregation studies. These assays are labor intensive, expensive, require repeated exposure of the CPB circuit to risk of contamination, and repeated withdrawal of blood from the circuit. For these reasons, the tests are run only intermittently and, therefore, there isa clinical need for a real-time diagnostic and continuous monitoring tool which could eliminate unnecessary operational delays while facilitating decision making during surgery and at post-operative stages. Light scattering-based sensing of blood condition in vivo is proposed as a method to monitor a patient's coagulation status on a real-time basis with a continuous or intermittent read-out available to the physician in much the same way as pulse oximetry, electrocardiogram, and respiratory and blood pressure monitors. The long term goal of this project is to overcome the limitations of the current blood coagulation and viscosity tests performed in emergency room and intensive care unit settings. For this purpose, we propose to develop an optical measurement technology based on dynamic scattering of low-coherence light. The optical sensing will be implemented using optical fiber technology which allows for integration with existing vascular access devices. Furthermore, dynamic light scattering measurements are fast and can be performed continuously enabling real-time monitoring functions. If successful operation in clinical environment is demonstrated, this technology may enable improved safety during and following surgery, thus decreasing risk of thrombosis, post-operative bleeding, and blood donor exposures.